On Sat, Apr 7, 2012 at 12:08 PM, TP <paratribulati...@free.fr> wrote: > Hello, > > In a module I am writing, I would like to use FlexibleInstances and > OverlappingInstances. > But I get errors, so I am trying to reproduce the problems on a smaller > program: >
Is your actual issue with Showing a list? If so, you might be better off using the 'showList' member of the 'Show' typeclass: instance Show Foo where show x = ... showList xs = ... Then your 'showList' method will be called when 'show' is called on a list of 'Foo' values. The first error is because 'map show l' is the wrong type - mapping show over a list will give you a list of strings, but 'show' must return a string. I think you could use 'concatMap' here. Other than that the only advice I can give is that I try my hardest to avoid OverlappingInstances. Antoine Antoine > -------------------------------------------- > {-# LANGUAGE FlexibleInstances, OverlappingInstances #-} > > data Foo = Foo Int > deriving ( Show ) > > instance Show [Foo] where > show [] = "[0]" > show l = map show l > > main = do > let l = [ Foo 1, Foo 2 ] > print l > -------------------------------------------- > > The first error I obtain is: > -------------------------------------------- > test_overlappinginstances.hs:7:19: > Couldn't match expected type `Char' with actual type `[Char]' > Expected type: a0 -> Char > Actual type: a0 -> String > In the first argument of `map', namely `show' > In the expression: map show l > -------------------------------------------- > > Where does this "Char" come from? How to solve this problem? > > The second error is: > -------------------------------------------- > test_overlappinginstances.hs:11:5: > Overlapping instances for Show [Foo] > arising from a use of `print' > Matching instances: > instance Show a => Show [a] -- Defined in GHC.Show > instance [overlap ok] Show [Foo] > -- Defined at test_overlappinginstances.hs:5:10-19 > -------------------------------------------- > > The overlap is ok ("overlap ok" does not appear if not using the pragma > OverlappingInstances), so it should work? > > Thanks in advance, > > TP > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe